Using amplitude ratio curves to evaluate cement sheath bonding in multi-string downhole environments

ABSTRACT

A method for evaluating a cement sheath in a wellbore, in some embodiments, comprises transmitting sonic or ultrasonic waves from a logging tool disposed in a wellbore, receiving reflected waves at the logging tool and recording waveforms based on the received waves, processing the waveforms to determine average absolute value amplitude data for each of a plurality of zones, and determining a number using average absolute value amplitude data for a first of the plurality of zones and average absolute value amplitude data for a second of the plurality of zones.

BACKGROUND

A cased wellbore typically possesses an annular space between the casingand the formation wall that is permanently sealed by being filled withcement. This layer of cement is typically referred to as a “cementsheath.” A properly formed cement sheath should fill all or nearly allof the annular space and should bond tightly to the casing and theformation.

Both sonic and ultrasonic waveforms have been used to evaluate thequality of this cement bond from a logging tool inside the casing. Thelogging tool, which may have one or more sonic or ultrasonic receiversand one or more sonic or ultrasonic transmitters, is lowered into awellbore and measurements are taken at various depths. Sonic orultrasonic waves are transmitted from the logging tool in the wellbore,and reflected waves from the casing, cement, and formation are received,recorded, processed, and interpreted to evaluate the presence andquality of the cement sheath and bond in the annular space between thecasing and the formation wall.

Processing received waveforms to produce cement bond logs, however, is ahighly subjective skill that is prone to substantial error and variationamong different interpreters. In addition, the cement bond logs that areproduced only provide bonding information pertaining to limited portionsof the cement sheath—that is, they provide an incomplete and oftenmisleading picture of the true cement bonding status of the sheath as awhole. Further, it is challenging to produce accurate cement bond logsin wellbores with multiple concentric casing strings where the cementsheath being evaluated is disposed outside of the outer casing string,since the concentric casing strings may interfere with the sonic orultrasonic waves transmitted to the cement sheath and reflected backfrom the cement sheath. Thus, methods and systems for generatingaccurate and consistently reproducible cement bond logs that visualizemost or all of the cement sheath, including in multi-stringenvironments, are desired.

BRIEF DESCRIPTION OF THE DRAWINGS

Accordingly, there are disclosed in the drawings and in the followingdescription various methods and systems for using amplitude ratio curvesto evaluate cement sheath bonding in multi-string downhole environments.In the drawings:

FIG. 1 is a schematic view of a well logging system in accordance withembodiments.

FIG. 2 is a perspective view of a cement bond logging tool according toembodiments.

FIG. 3 is a flow chart of a method for evaluating bonding of a cementsheath in a wellbore and creating a cement bond log in accordance withembodiments.

FIG. 4A is a graph of four recorded waveforms at four different depthstaken in a shallower section of a wellbore characterized by free(“unbonded”) pipe, in accordance with embodiments.

FIG. 4B is a graph of four recorded waveforms at four different depthstaken in a deeper section of a wellbore characterized by bonded pipe, inaccordance with embodiments.

FIG. 5 is a graph showing the mathematical derivative plots of eachwaveform of FIG. 4A for determining minima and maxima, according toembodiments.

FIG. 6A is a plot of the magnitudes of the peaks and troughs of the fourrecorded waveforms of FIG. 4A, in accordance with embodiments.

FIG. 6B is a plot of the magnitudes of the peaks and troughs of the fourrecorded waveforms of FIG. 4B, in accordance with embodiments.

FIG. 7A is a plot of the absolute values of the magnitudes of the peaksand troughs of the four recorded waveforms of FIG. 4A, in accordancewith embodiments.

FIG. 7B is a plot of the absolute values of the magnitudes of the peaksand troughs of the four recorded waveforms of FIG. 4B, in accordancewith embodiments.

FIG. 8A is a plot of superpositioned points of the absolute value pointsof FIG. 7A, in accordance with embodiments.

FIG. 8B is a plot of superpositioned points of the absolute value pointsof FIG. 7B, in accordance with embodiments.

FIG. 9 is a combined plot of all eight series of superpositionedabsolute value magnitudes of FIGS. 8A and 8B, showing the data dividedaccording to naturally occurring zones, in accordance with embodiments.

FIG. 10A is a plot of the absolute value magnitudes of FIG. 8A averagedaccording to the zones of FIG. 9, shown both as amplitude curves versusdepth and as grayscale logs versus depth, in accordance withembodiments.

FIG. 10B is a plot of the absolute value magnitudes of FIG. 8B averagedaccording to the zones of FIG. 9, shown both as amplitude curves versusdepth and as grayscale logs versus depth, in accordance withembodiments.

FIG. 11A is a perspective view of a cement bond logging tool, inaccordance with embodiments.

FIG. 11B is a cross-sectional view of a multi-directional cement bondlogging tool disposed within a wellbore, in accordance with embodiments.

FIG. 12 is a flowchart of a method usable to produce grayscale logs fordifferent zones, in accordance with embodiments.

FIG. 13 is a plot of the waveforms recorded by each segment of amulti-directional antenna after having been processed as described withrespect to FIGS. 1-10B, shown as grayscale logs versus depth, inaccordance with embodiments.

FIG. 14 shows the grayscale logs of FIG. 13 rearranged to facilitateinterpretation, in accordance with embodiments.

FIG. 15 shows multiple grayscale logs versus depth, each logrepresenting the amplitude of waveforms received across a differentshell, in accordance with embodiments.

FIG. 16 is a flowchart of a method usable to produce ratio or differenceplots to facilitate identification of strong cement bond locations andpoor cement bond locations, in accordance with embodiments.

FIG. 17 is a set of amplitude and ratio plots produced by performing themethod of FIG. 16, in accordance with embodiments.

FIG. 18 is another set of amplitude and ratio plots produced byperforming the method of FIG. 16, in accordance with embodiments.

It should be understood, however, that the specific embodiments given inthe drawings and detailed description thereto do not limit thedisclosure. On the contrary, they provide the foundation for one ofordinary skill to discern the alternative forms, equivalents, andmodifications that are encompassed together with one or more of thegiven embodiments in the scope of the appended claims.

DETAILED DESCRIPTION

Disclosed herein are techniques for generating accurate, consistentlyreproducible cement bond logs that visualize most or all of the cementsheath in the annular space of a wellbore in multi-string environments.The techniques include the use of a multi-directional cement bondlogging tool that transmits sonic or ultrasonic waves in a radialfashion away from the tool. The waves encounter various objects as theypropagate through the areas surrounding the tool, such as open spacecontaining fluid, the casing(s), the cement sheath (or other material)in the annular space between a casing and the formation, and theformation itself. Each of these areas reflects some portion of the wavesback to the multi-directional logging tool, which captures the reflectedsignals from multiple directions and records them as time-domainwaveforms.

Each received waveform is processed (as described herein), resulting inwave amplitude values that indicate—among other things—whether theannular space between a casing and the formation contains a propercement bond. This is possible because waveforms have differentsignatures when the annular space is filled with fluid (free pipe) orsolid (cement). The free pipe signature includes higher amplitudes, alow rate of attenuation and a consistent waveform. When the annularspace is filled with a solid material the amplitude of the waveform isreduced, the attenuation of the same waveform is increased, and thewaveforms are not consistent.

The techniques entail assigning each of the processed amplitude values adifferent color, grayscale shade or intensity. The colors, shades orintensities for all amplitude values of waveforms received from alldirections are used to form a composite image that indicates the degreeof cement bonding in various areas of the annular space. The compositeimage is further developed by repeating the process at multiple depthsof the wellbore. In this way, a color-, grayscale- or intensity-codedcomposite image may be formed that gives a 360-degree representation ofthe degree of cement bonding throughout the depth of the entirewellbore, thereby facilitating the identification of poor cement bondsin areas that may have otherwise gone undetected.

The following description is divided into multiple parts. The firstpart, entitled “Unidirectional Waveform Processing,” primarily describesthe manner in which signals received by the tool from a single directionat various wellbore depths are processed. The result of thisunidirectional waveform processing is a color-, grayscale- orintensity-coded image that indicates the degree of cement bondingpresent in a limited portion of the annular space. The second part,entitled “Multidirectional Waveform Processing,” expands the conceptsdescribed in the first part by describing how signals received frommultiple directions at multiple wellbore depths are jointly processed.The result of multidirectional waveform processing is a color-,grayscale- or intensity-coded image that indicates the degree of cementbonding throughout the annular space at some or all depths of thewellbore—essentially, a visualization of the cement bond for nearly theentire wellbore. Finally, the third part—entitled “Using Ratios andDifferences in Multi-String Environments”—presents a technique that maybe used alone or in conjunction with other techniques described hereinto evaluate cement bonding in environments having multiple concentriccasing strings.

Unidirectional Waveform Processing

FIG. 1 is a schematic block diagram of a well logging system 10. Alogging cable 11 suspends a sonde 12 in a wellbore 13. Wellbore 13 isdrilled by a drill bit on a drill string and is subsequently lined withcasing 19 and an annular space 20 that contains, e.g., cement (known inthe art as a “cement bond”). Wellbore 13 can be any depth, and thelength of logging cable 11 is sufficient for the depth of wellbore 13.For illustrative purposes and as described in greater detail below, ashallow portion 13 a of wellbore 13 lacks a proper cement bond in theannular space 20, while a deeper portion 13 b of wellbore 13 has a goodcement bond in the annular space 20.

Sonde 12 generally comprises a protective shell or housing that is fluidtight and pressure resistant and that enables equipment inside the sondeto be supported and protected during deployment. Sonde 12 encloses oneor more logging tools that generate data useful in analyzing wellbore 13or in determining various material properties of the formation 21 inwhich wellbore 13 is disposed.

In some embodiments, a cement bond logging tool 14 (e.g., amulti-directional logging tool, such as a radial antenna, apitch-and-catch transducer, or a pulse echo transducer) is provided, asdescribed below with respect to FIG. 2, for determining how well thecement sheath within the annular space 20 bonds with the casing 19 andthe wall of formation 21. Other types of tools may also be included insonde 12, such as a gamma ray tool 18. Sonde 12 may also enclose a powersupply 15. Output data streams from cement bonding logging tool 14 andgamma ray tool 18 may be provided to a multiplexer 16 housed withinsonde 12. Sonde 12 may include a communication module 17 having anuplink communication device, a downlink communication device, a datatransmitter, and a data receiver.

Logging system 10 includes a sheave 22 that is used to guide the loggingcable 11 into wellbore 13. Cable 11 is spooled on a cable reel 23 ordrum for storage. Cable 11 couples with sonde 12 and is spooled out ortaken in to raise and lower sonde 12 in wellbore 13. Conductors in cable11 connect with surface-located equipment, which may include a DC powersource 24 to provide power to tool power supply 15, a surfacecommunication module 25 having an uplink communication device, adownlink communication device, a data transmitter and also a datareceiver, a surface computer 26 (or, more generally, any suitable typeof processing logic), a logging display 27 and one or more recordingdevices 28. Sheave 22 may be coupled by a suitable means to an input tosurface computer 26 to provide sonde depth measuring information. Thesurface computer 26 comprises processing logic (e.g., one or moreprocessors) and has access to software (e.g., stored on any suitablecomputer-readable medium housed within or coupled to the computer 26)and/or input interfaces that enable the computer 26 to perform, assistedor unassisted, one or more of the methods and techniques describedherein. The computer 26 may provide an output for a logging display 27and a recording device 28. The surface logging system 10 may collectdata as a function of depth. Recording device 28 is incorporated to makea record of the collected data as a function of depth in wellbore 13.

In some embodiments, processing logic (e.g., one or more processors) andstorage (e.g., any suitable computer-readable medium) may be disposeddownhole within the sonde 12 and may be used either in lieu of thesurface computer 26 or in addition to the computer 26. In suchembodiments, storage housed within the sonde 12 stores data (such asthat obtained from the logging operations described herein), which maybe downloaded and processed using the surface computer 26 or othersuitable processing logic once the sonde 12 has been raised to thesurface (e.g., in “slickline” applications). In some embodiments,processing logic housed within the sonde 12 may process at least some ofthe data stored on the storage within the sonde 12 before the sonde 12is raised to the surface.

FIG. 2 is a perspective view of a cement bond logging tool 14 accordingto one or more embodiments. Cement bond logging tool 14 may include asource transmitter 40 and two or more receivers 41, 42, which may bearranged in a pitch and catch configuration. That is, source transmitter40 may be a pitch transducer, and receivers 41, 42 may be near and farcatch transducers spaced at suitable near and far axial distances fromsource transmitter 40, respectively. In such a configuration, sourcepitch transducer 40 emits sonic or ultrasonic waves while near and farcatch transducers 41, 42 receive the sonic or ultrasonic waves afterreflection from the wellbore fluid, casing, cement and formation andrecord the received waves as time-domain waveforms. Because the distancebetween near catch transducer 41 and far catch transducer 42 is known,differences between the reflected waveforms received at each catchtransducer 41, 42 provide information about attenuation that can becorrelated to the material in the annular wellbore region, and theyallow a circumferential depth of investigation around the wellbore.

The pitch-catch transducer pairing may have different frequency,spacing, and/or angular orientations based on environmental effectsand/or tool design. For example, if transducers 40-42 operate in thesonic range, spacing ranging from three to fifteen feet may beappropriate, with three and five foot spacing being common. Iftransducers 40-42 operate in the ultrasonic range, the spacing may beless.

Cement bond logging tool 14 may include, in addition or as analternative to transducers 40-42, a pulsed echo ultrasonic transducer43. Pulsed echo ultrasonic transducer 43 may, for instance, operate at afrequency from 80 kHz up to 800 kHz. The optimal transducer frequency isa function of the casing size, weight, mud environment and otherconditions. Pulsed echo ultrasonic transducer 43 transmits waves,receives the same waves after they reflect off of the casing, annularspace and formation, and records the waves as time-domain waveforms.

FIG. 3 is a flow chart of a peak analysis method 50 for evaluatingbonding of cement in a wellbore and creating a cement bond log accordingto one or more embodiments of the invention. Peak analysis method 50 maybe applied to any reflected waveform received from the structures in thewellbore that are adjacent to the cement bond logging tool 14. That is,peak analysis method 50 can use—without limitation—standard sonic, pulseecho ultrasonic, and/or pitch and catch ultrasonic waveforms and processsuch waveforms, regardless of the waveform type or method of generation,using identified peaks and troughs to determine the type andcharacteristic of the material in the wellbore annular space. Peakanalysis method 50 is a visual method to determine cement placement inthe wellbore annular space, not only near the casing wall, but also inareas away from the casing wall, which have historically been difficultto evaluate.

For illustration of peak analysis method 50, eight illustrative,recorded, reflected waveforms are presented. Referring to FIGS. 1, 3,4A, and 4B, at step 51, a plurality of reflected waveforms 60 at varyingwellbore depths are recorded, such as by using cement bonding loggingtool 14. FIG. 4A is a graph of four recorded waveforms 60 a at fourdifferent depths taken in a shallower section 13 a of wellbore 13characterized by free, or unbonded, pipe, and FIG. 4B is a graph of fourrecorded waveforms 60 b at four different depths taken in a deepersection 13 b of wellbore 13 characterized by bonded pipe. It can be seenthat there is a different waveform response at the free pipe zone andthe bonded pipe zone. In the free pipe zone 13 a, the amplitudes ofwaveforms 60 a are relatively high. Because cement in the annular space20 attenuates the waves in bonded section 13 b, waveforms 60 b arecharacterized by a lower amplitude. The increased amplitude of waveforms60 b to the right of the chart are attributable to the response offormation 21.

At step 52, for each of the recorded waveforms 60, location of amplitudemaxima and minima, or peaks and troughs, are identified. In some cases,maxima and minima can be readily identified by visually inspecting thewaveforms 60, but such identification is difficult and inaccurate withcomplex waveforms. Accordingly, in embodiments, maxima and minima areidentified by taking the mathematical derivative of each waveform 60.FIG. 5 is a graph showing the mathematical derivative plots 63 of eachwaveform 60 a of FIG. 4A. Each instance where derivative slope plot 63changes sign (going from positive to negative or negative to positive)corresponds to a peak or trough of the corresponding waveform 60 a. Twosuch points 64, 65 are labeled on FIG. 5 for illustration, whichcorrespond to peaks 61 and troughs 62 of waveforms 60 a. Although notillustrated directly, the same procedure is performed with each waveform60 b of FIG. 4B.

At step 53, for each maxima and minima identified in step 52, theabsolute values of the amplitudes of waveforms 60 are identified. Step53 is illustrated in two stages. FIGS. 6A and 6B are plots of themagnitudes 66 of the peaks and troughs of waveforms 60 a, 60 b of FIGS.4A, 4B, respectively. The time value corresponding to each identifiedmaxima and minima (e.g., points 64, 65 of FIG. 5) are used to extractthe magnitude values of waveforms 60 at the same times. However,magnitudes are plotted as a numbered series of points (E1, E2 . . .E_(n)) rather than based on the timescale where E1 corresponds to theamplitude of the first arrival. For instance, magnitude values 67, 68correspond to peaks 61 and troughs 62 of waveforms 60 a. As shown, themagnitude points 66 may be connected by straight lines to aid viewing.The second stage of step 53 is illustrated by FIGS. 7A and 7B, which areplots of the absolute values 69 of the magnitude values 66. For example,absolute value points 72 and 74 correspond to magnitude values 67, 68 ofFIG. 6A. Again, the absolute value points 69 may be connected bystraight lines to aid viewing. By using absolute values, both thepositive and negative peaks of the entire waveform 66 are consideredtogether in a simplified manner.

From FIGS. 7A and 7B, it is possible to identify some general trends inthe data of each waveform, and various natural groupings or sectionsappear. Stacking, or superpositioning, the absolute value points 69further highlights these groupings. FIGS. 8A and 8B illustrate thisstep. FIGS. 8A and 8B are plots of superpositioned points 76 of theabsolute value points 69 of FIGS. 7A and 7B. For example, a first series77 of absolute value magnitudes 76 a of FIG. 8A is the same as series 70of FIG. 7A. A second series 78 of absolute value magnitudes 76 a of FIG.8A is the summation of corresponding absolute value magnitudes 69 a ofseries 70 and 71 of FIG. 7A. A third series 79 of absolute valuemagnitudes 76 a of FIG. 8A is the summation of corresponding absolutevalue magnitudes 69 a of series 70, 71 and 73 of FIG. 7A. Likewise, thefourth series 80 of absolute value magnitudes 76 a of FIG. 8A is thesummation of corresponding absolute value magnitudes 69 a of series 70,71, 73, and 75 of FIG. 7A. FIG. 8B is generated from FIG. 7B in the samemanner. The order in which the absolute value magnitudes are stacked isnot critical, as it merely serves to highlight natural trends in thedata.

Using the above sequence of steps 51-53, various patterns begin toemerge from both the free and bonded sections 13 a, 13 b of wellbore 13.A person of ordinary skill in the art will recognize that there are fouror more distinct areas, breaks, or zones in the waveform response.Accordingly, at step 54 (and as described below with respect to FIG. 9),the series of points (E1, E2 . . . E_(n)) is analyzed and sorted basedaccording to these naturally occurring divisions. Each zone can beadjusted or shifted based on the waveform response, casing size, casingweight, cement properties and other environmental conditions of thewell. In the example of the present disclosure five zones are evident,but a greater or lesser number of zones may be appropriate for a givenset of waveforms.

FIG. 9 is a combined plot of all eight series of stacked absolute valuemagnitudes 76 of FIGS. 8A and 8B. The vertical lines show where ananalyst may examine the waveform response data and divide the series ofpoints into four or more zones according to step 54. Zone 1 correspondsto casing signal arrivals and Zone 5 corresponds to formation signalarrivals. Zones 2-4 therefore encompass the annular area between thecasing and the formation, with Zones 2 and 4 being somewhat influencedby the casing and formation, respectively.

In some embodiments, the grouping of zones is based on the shape of thestacked waveforms as shown in FIG. 9. In such embodiments, zones aregrouped according to slope changes of the stacked waveforms. Forexample, as shown in FIG. 9, dramatic slope changes may be observed atE3, E6, E8, and E10, which are the points that divide Zones 1-5. Thethreshold that a particular slope change must exceed to qualify as azone dividing point may be set as suitable and desired. In otherembodiments, the peak values themselves may be used to divide thewaveforms into zones—for instance, the low and high peaks may bedesignated as the points at which the zones are divided. Referring toFIG. 9, for instance, point E3 is the highest peak that occurs prior tothe next low peak (i.e., E6), so E3 may be designated as the dividingline between Zones 1-2. Peak E4 is of a value similar to that of E3, soE4 may be designated as the dividing line between Zones 1-2 in lieu ofE3. Similarly, peak E6 is the lowest value occurring prior to the nexthigh point (i.e., E8), so E6 may be designated as the dividing linebetween Zones 2-3.

In preferred embodiments, each zone has a minimum of two peaks, althoughthis is not required. In some embodiments, waveforms with the highestamplitudes—which generally correspond to free pipe areas—may have zonesthat are designated separately from those waveforms with the lowestamplitudes, which generally correspond to cement bonded areas. Thus, forinstance, referring to FIG. 9, in these embodiments the determination ofzones for the top four waveforms—which have the highest amplitudes—maybe performed separately from the determination of zones for the bottomfour waveforms, which have the lowest amplitudes. In some embodiments,zones may be selected such that each zone corresponds to a separate“wave” in the waveforms. For instance, in such embodiments, Zones 1-2would be separated at E6; Zones 2-3 would be separated at E9, and soforth. Any of these techniques may be used, or alternative, suitabletechniques may be used instead. Regardless of the techniques used, inpreferred embodiments, the zones that are to be plotted (as describedbelow) are selected such that they do not include formation signals,which are typically signals of very high amplitude relative to othersignal arrivals from the casing string and annular space (e.g., in FIG.9, the formation signal arrivals likely correspond to peaks E16 andonward).

Once the zones are selected at step 54, the average amplitude for eachwaveform within each zone is determined at step 55. This is accomplishedby calculating, for each zone, the mean value of each waveform's peaks.Thus, for instance, referring to FIG. 9, each of the waveforms presentin Zone 1 may be averaged using the peaks E1, E2 and E3 present inZone 1. This results in multiple mean values for Zone 1, each mean valuecorresponding to a different waveform. Similarly, peaks E4, E5 and E6are used to calculate the mean value for Zone 2. In some embodiments,the peaks that straddle two zones (e.g., peaks E3, E6) may be used tocalculate the mean values of both zones that are straddled.Alternatively, in some embodiments, such a peak—like peak E3—is used tocalculate the mean value for Zone 2 and not for Zone 1. In someembodiments, the average amplitude of Zone 1 peaks are calculated twice,once with the 1st peak (E1) and once without. Because E1 values areinherently smaller than E3 values, removing the first set of values fromthe calculation of the arithmetic mean facilitates comparison of Zone 1to the other zones.

In existing methods, pipe amplitude is calculated from the amplitude ofthe E1 arrival and then normalized to a certain value based on casingsize and weight. With peak analysis method 50, however, the E1 arrivaldata is not used in at least some embodiments. Accordingly, the waveformdata is not normalized to a certain value for pipe size and weight butrather is optionally normalized to the free pipe value for the casing inquestion. Thus, the highest amplitude of Zone 1 is determined to be 100%free (assuming that there is a free pipe section), and all the otheramplitudes for the other zones are normalized accordingly. Statedanother way, the end result of the above-described steps is a number ofdifferent amplitude curves determined by the natural break in thewaveform response and normalized to a free pipe value of unity. Althoughnot required, such normalization allows comparison of the waveformresponses to a known point of reference.

The average amplitudes for each zone, whether normalized or not,indicate a quality of cement bonding. However, to facilitateinterpretation of such amplitude data, a plot with color, grayscale, orintensity mapping may be used to visually show the amplitude values ofeach zone at each depth in the wellbore. For instance, colors may beused ranging from a black for low amplitudes grading to lighter colorsthen finally to a light blue which grades to a dark blue at 100 percentfree pipe amplitude. A grayscale scheme with various shades may be usedin lieu of such colors, as may a scheme employing an intensity map.

FIGS. 10A and 10B illustrate such a plot (or “cement bond log”). FIGS.10A and 10B are logs of the absolute value amplitudes of FIGS. 8A and8B, respectively, averaged according to the zones of FIG. 9. FIGS. 10Aand 10B show the zonal averaged absolute value amplitude data, both asamplitude curves versus depth and as color-, grayscale- orintensity-maps versus depth. A left-hand graph 80 includes fouramplitude curves 82 (labeled AV1PEAKW, AV2PEAK, AV3PEAK, AV4PEAK) of thezonal averaged absolute value amplitude data versus depth. To theimmediate right of graph 80, curves 82 are re-plotted as grayscale logs81 a-81 d, which show the grayscale coding of the above curves in thesame order, with a scale from 0 to 100 and with depth shown on thevertical axis. Log 81 a corresponds to Zone 1; log 81 b corresponds toZone 2; log 81 c corresponds to Zone 3; and log 81 d corresponds to Zone4. Thus, log 81 a represents amplitudes of waves received from thecasing, log 81 d represents amplitudes of waves received from theformation, and logs 81 b and 81 c represent amplitudes of waves receivedfrom the annular space between the casing and the formation. The darkershades indicate good cement bond (as shown in FIG. 10B, whichcorresponds to the lower part 13 b of the wellbore), and the lightestshades indicate poor cement bond (as shown in FIG. 10A, whichcorresponds to the upper part 13 a of the wellbore). Other schemes,including color and intensity mapping, may be used as appropriate.

The foregoing technique is useful for mapping the entire depth of a wellalong a single vertical column. That is, the technique is able to createa visual depiction of the waveform amplitudes received from the fluid,casing, annular space, and formation for the entire depth of thewell—but only for unidirectional waveforms. Thus, although the mappingmay accurately indicate the quality of cement bonding in the directionin which the tool antenna is oriented, it does not provide informationabout the cement bonding in directions where the tool antenna is notoriented.

Multidirectional Waveform Processing

Accordingly, in some embodiments, a multi-directional cement bondlogging tool may be used to transmit sonic and/or ultrasonic waves toand receive sonic and/or ultrasonic waves from multiple directions ateach depth, thereby providing composite images that visualize asubstantial portion of the annular space at each depth. FIG. 11Aprovides a perspective view of a multi-directional cement bond loggingtool 14. The tool 14 comprises a multi-directional transmitter 89 andmultiple directional receivers 87, 88. In some embodiments, amulti-directional transceiver may be used in lieu of separatetransmitters and receivers. The remainder of this discussion assumes theuse of transceiver embodiments, although techniques described herein maybe adapted for use with any suitable logging tool configuration. FIG.11B provides a cross-sectional view of a multi-directional cement bondlogging tool (e.g., radial antenna) 14 disposed within a wellbore 13.The tool 14 is disposed within a portion 99 of the wellbore thatcontains fluid or another known material. Concentrically adjacent to theportion 99 is the casing string 100. Concentrically adjacent to thecasing string 100 is the annular space 101—an area of particularinterest because it contains a cement layer, the quality of which thetool 14 is intended to ascertain. Concentrically adjacent to the casingstring 100 is the formation 102.

The logging tool 14 comprises multiple transceivers (or “segments”) 90a-90 h disposed in a radial fashion about the circumference of the tool14, although the scope of this disclosure is not limited to any specificnumber of segments or any particular radial positioning of the segments.Using more segments will provide additional data from additional partsof the annular space and cement layer, thereby producing an image of theannular space and the cement bond layer that has greater resolution. Incontrast, using fewer segments provides fewer data points and thusproduces an image of the annular space and cement layer that has poorerresolution. Each of the segments 90 a-90 h is capable of transmittingsonic and/or ultrasonic waves in the general direction indicated bydashed lines 91-98, respectively. Similarly, each of the segments 90a-90 h is adapted to receive sonic and/or ultrasonic waves from thegeneral direction indicated by dashed lines 91-98, respectively. Again,as explained, any suitable tool configuration may be used.

In practice, each of the segments 90 a-90 h transmits signals in thedirections of lines 91-98, respectively, and subsequently receivesreflected waves and records waveforms accordingly. The waveformsrecorded by segment 90 a are processed as described in detail above withrespect to FIGS. 3-10B, thereby producing a visual depiction of thewaveform amplitudes for multiple zones in the general direction ofdashed line 91. Stated another way, processing the waveforms recorded bysegment 90 a produces an image (e.g., coded using color, grayscale,intensity) that indicates the quality of cement bond in the portion ofthe annular space that coincides with dashed line 91. Tool 14, however,also comprises segments 90 b-90 h, each of which records its ownwaveforms, just like segment 90 a. These waveforms, when processed asdescribed above, produce images that indicate the quality of cement bondin the portions of the annular space that coincide with the dashed lines92-98. The net result of this process using segments 90 a-90 h is a setof eight images indicating the quality of cement bond at the portions ofthe annular space coinciding with the eight dashed lines 91-98. Statedanother way, the zone(s) that corresponds to the annular space (e.g.,Zone 3) is visualized at the points indicated by dashed lines 91-98. Thedata in these multiple images of the annular space zone(s) may beaggregated to produce a composite set of data (generally referred toherein as a “shell” because the data is obtained from numerous pointsaround the circumference of the zone(s)) that indicates the quality ofcement bond throughout the annular space 101 for the depth at which thewaveforms were recorded. The shell data may be modified using anysuitable interpolation technique (e.g., a linear averaging technique) todetermine appropriate averaged absolute value amplitude data for theportions of the zone(s) that are located in between the dashed lines91-98. By repeating this process at multiple depths throughout thewellbore, a composite shell image is formed to produce an indication ofthe quality of cement bond for the entire annular space at all depths ofthe wellbore—essentially, a visual depiction of the cement bond qualityfor the entire annular space 101.

FIG. 12 shows a flowchart of a method 105 usable to produce logs fordifferent shells. The method 105 first comprises transmitting sonicand/or ultrasonic signals in and receiving sonic and/or ultrasonicsignals from multiple directions (step 106). As explained with respectto FIG. 11, this step may be performed by a multi-directional cementbond logging tool 14, such as a multi-directional antenna. After thetool 14 records time-domain waveforms based on incoming sonic and/orultrasonic signals, the method 105 comprises processing each receivedwaveform using the techniques described above with respect to FIGS.3-10B (step 107). Performing step 107 results in averaged absolute valueamplitude data in each zone for each received waveform. FIG. 13 showseight grayscale logs 115-122, although other schemes, such as color orintensity mapping, also may be used. The logs 115-122 in FIG. 13correspond to waveforms recorded by the segments 90 a-90 h,respectively. In addition, each logs 115-122 contains multiple sub-logsa-d. Sub-logs a-d correspond to waveform data from Zones 1-4,respectively. Thus, for instance, log 115 a indicates the averagedabsolute value amplitude data in Zone 1 (e.g., the general area of thecasing string 100) for the waveform received from segment 90 a.Similarly, for example, log 121 c indicates the averaged absolute valueamplitude data in Zone 3 (e.g., the outer area of the annular space 101within which the cement layer is disposed) for the waveform receivedfrom segment 90 g. Although each iteration through method 105 processeswaveforms at a single depth of the wellbore, the logs 115-122 in FIG. 13plot depth on the y-axis to demonstrate how a complete set of logs mayappear for the entire depth of the wellbore.

As explained above, each of the dashed lines 91-98 in FIG. 11B indicatesthe general areas from which the waveform data of FIG. 13 is obtained.Dashed line 91 corresponds to the data shown in log 115, while dashedline 92 corresponds to the data shown in log 116, and so on. For thisreason, it is possible to rearrange the logs of FIG. 13 in such a waythat they provide cross-sectional views of the amplitude data from thewellbore. Stated another way, and referring to FIG. 11B, the data fordashed line 91—which corresponds to log 115—can be grouped with the datafor dashed line 95—which corresponds to log 119. Grouping the logs ofdashed lines that are separated by 180 degrees in this way provides across-sectional view of the logs 115-122, thereby making the logs easierto understand and interpret.

FIG. 14 shows such a rearrangement of logs 115-122. Specifically, logs115 and 119 are grouped together (shown as log 125) because theycorrespond to dashed lines 91 and 95. Similarly, logs 116 and 120 aregrouped together (shown as log 126) because they correspond to dashedlines 92 and 96. Logs 117 and 121 are grouped (shown as log 127) becausethey correspond to dashed lines 93 and 97, and logs 118 and 122 aregrouped (shown as log 128) because they correspond to dashed lines 94and 98. As shown in FIG. 14, logs 115 and 119 are oriented in such a waythat the open wellbore space 99 is in the middle, Zone 1 data (i.e.,logs 115 a and 119 a) is immediately adjacent to the open space 99, Zone2 data (i.e., logs 115 b and 119 b) is immediately adjacent to the Zone1 data, Zone 3 data (i.e., logs 115 c and 119 c) is immediately adjacentto the Zone 2 data, and Zone 4 data (i.e., logs 115 d and 119 d) isimmediately adjacent to the Zone 3 data. In this way, track 1 shows whata cross-sectional view of the averaged absolute value amplitude datataken at dashed lines 91 and 95 would look like for the entire wellboreat all depths, starting with the open space 99 and moving outward withlogs 115 a and 119 a representing data roughly corresponding to thecasing string 100, logs 116 a and 120 a representing data roughlycorresponding to the inner portion of the annular space 101, logs 117 aand 121 a representing data roughly corresponding to the outer portionof the annular space 101, and logs 118 a and 122 a representing dataroughly corresponding to the area of the annular space 101 immediatelyadjacent to the formation 102. Logs 126-128 are similarly arranged.

Referring again to FIG. 12, the method 105 next comprises aggregatingthe averaged absolute value amplitude data for each zone across allreceived waveforms into shells (step 108), which were briefly describedabove. For instance, Zone 1 data across all waveforms—i.e., datarepresented by logs 115 a, 116 a, 117 a, 118 a, 119 a, 120 a, 121 a and122 a—are aggregated into a shell. These data were collected from thecasing string 100 at the points coincident with dashed lines 91-98. Theareas of string 100 not coincident with dashed lines 91-98, however, areunaccounted for and are not represented in the shell. Accordingly, anysuitable interpolation technique (e.g., a linear averaging technique) isused to determine data values for the areas of the string 100 betweenthe dashed lines 91-98. In this way, the shell presents a more completepicture of the data for the entire string 100. A similar process isperformed for Zone 2 data across all waveforms—that is, datacorresponding to the inner portion of the annular space 101; for Zone 3data across all waveforms, which correspond to the outer portion of theannular space 101; and for Zone 4 data across all waveforms, whichcorrespond to the area of the annular space 101 that is immediatelyadjacent to the formation 102. The aggregated data is then used tocreate a composite image of cement bond quality for each shell (step109). Specifically, the images are generated using the data from logs115-122 and any of a variety of suitable interpolation techniques; eachimage corresponds to a different shell and represents the averagedabsolute value amplitude data for that shell. Steps 106-109 are thenrepeated at numerous depths within the wellbore 13 (step 110), andadditional data obtained from each depth is added to the composite shellimages generated in step 109 to visualize the entire wellbore. Thenumber of depths at which the process 105 is performed may vary, but inthe instance that the averaged absolute value amplitude data between anytwo depths in missing, any suitable interpolation technique may be usedto determine that data, and the data may be added to the composite shellimage in question.

FIG. 15 shows a set of such composite images (or composite logs) foreach shell. Image 130 is a grayscale image of the averaged absolutevalue amplitude data for the shell that corresponds to Zone 1—e.g., thecasing string. It is formed using all available Zone 1 data—that is,data in logs 115 a, 116 a, 117 a, 118 a, 119 a, 120 a, 121 a and 122a—and any of a variety of suitable interpolation techniques. Similarly,image 131 is a grayscale image of the averaged absolute value amplitudedata for the shell that corresponds to Zone 2—e.g., the inner area ofthe annular space. It is formed using all available Zone 2 data—that is,data in logs 115 b, 116 b, 117 b, 118 b, 119 b, 120 b, 121 b and 122b—and a suitable interpolation technique. Image 132 is a grayscale imageof the averaged absolute value amplitude data for the shell thatcorresponds to the outer portion of the annular space 101. It is formedusing all available Zone 3 data—that is, data in logs 115 c, 116 c, 117c, 118 c, 119 c, 120 c, 121 c and 122 c—and a suitable interpolationtechnique. Finally, image 133 is a grayscale image of the averagedabsolute value amplitude data for the area of the annular space 101immediately adjacent to the formation 102. It is formed using allavailable Zone 4 data—that is, data in logs 115 d, 116 d, 117 d, 118 d,119 d, 120 d, 121 d and 122 d—and a suitable interpolation technique.Images 131-133 may be of greatest interest, because they are visualrepresentations of the cement layer disposed within the annular space101. Lighter shades represent high averaged absolute value amplitudedata and thus poor cement bonds (or the absence of cement altogether),while darker shades represent low averaged absolute value amplitude dataand thus strong cement bonds.

This discussion describes Zones 1-4 (and their corresponding shells) asif they precisely coincide with the casing 100, inner annular space 101,outer annular space 101, and the annular space 101 immediately adjacentto the formation 102, respectively. In some embodiments, however, thismay not be the case. The recorded waveforms may, in some cases, bedivided into zones (FIG. 4, step 54; FIG. 10) such that a single zone(and the corresponding shell) coincides with multiple features in thewellbore. For instance, instead of corresponding only to the casingstring 100, Zone 2 may correspond to some or all of the casing string100 and some or all of the annular space 101. In addition, while theembodiments have been described above in the context of four zones andfour shells, any suitable number of zones and shells may be used,according to the discretion of one of ordinary skill in the art.Further, the techniques described thus far have been in thesingle-casing string context. The same techniques, however, also may beused to evaluate cement in annular spaces behind multiple casingstrings. In such embodiments, waveforms may be divided into more thanfour zones so that each annular space that contains cement is assignedto at least one zone. Further still, although in the foregoingembodiments sonic and/or ultrasonic signals are transmitted and receivedusing the multi-directional logging tool 14 shown in FIGS. 11A-11B, insome embodiments, other suitable tools are used, such as the pitch andcatch transducers 40-42 or pulse echo transducer 43 of FIG. 2. Thesetransducers, though unidirectional, may be mounted on a wireline suchthat they spin about its axis, thereby enabling the transducers totransmit sonic and/or ultrasonic signals in and receive sonic and/orultrasonic signals from multiple directions.

Using Ratios and Differences in Multi-String Environments

FIG. 16 is a flowchart of a method that may be used to evaluate thequality of cement present in annular spaces behind one or more casingstrings. Although it is described in the context of a unidirectionalcement bond logging tool, it may also be performed in embodimentsdeploying a multi-directional logging tool. In addition, although thetechnique is described in the context of a two-casing stringenvironment, the technique also may be performed in downholeenvironments with a single casing string or more than two casingstrings. The method generally entails determining the averaged absolutevalue amplitude data for each of a plurality of zones of a waveform asdescribed above with respect to FIGS. 3-10B, and then determiningnumerical ratios or differences between the data for the zones. Thisprocess is repeated at multiple depths. The calculated ratios ordifferences are then plotted as a function of depth and are analyzed toidentify areas of an annular space (e.g., an annular space behind theouter of two concentric casing strings) that possess strong cementbonding and those than have weaker cement bonding.

The method 140 begins with transmitting and receiving sonic and/orultrasonic signals at multiple depths of a wellbore (step 141), and itfurther comprises processing each recorded waveform according to thetechniques described above with respect to FIGS. 3-10B (step 142). Theresult of step 142 is a set of average absolute value amplitude data foreach of a plurality of zones and at multiple wellbore depths. The method140 then comprises calculating ratios (or, alternatively, calculatingdifferences) among the average absolute value amplitude data fordifferent zones at each depth (step 143). For instance, if the averageabsolute value amplitude for a hypothetical Zone 1 is 10,000 at a depthof 500 feet and the average absolute value amplitude for a hypotheticalZone 3 is 2,000 at the same depth of 500 feet, the ratio of Zone 1 toZone 3 at 500 feet is 5. Such ratios between Zone 1 and Zone 3 data maybe calculated throughout multiple depths of the wellbore, therebyproducing a Zone 1-to-Zone 3 ratio curve for some or all depths of thewellbore. The ratio curve is then plotted (step 144). Such ratio curvesmay be produced for any combination of zones and is not limited to Zones1 and 3. Difference curves (i.e., performed by subtracting the averageabsolute value amplitude data for different zones at a common depth) maybe generated and plotted in a similar fashion.

Each ratio curve or difference curve is plotted with one or morecompanion curves that indicates the ratios or differences between twoother zones besides the zones that have already been plotted. Forinstance, a single ratio plot may contain both a Zone 1-to-Zone 2 ratiocurve and a Zone 2-to-Zone 3 ratio curve. Similarly, a single differenceplot may contain both a Zone 1-Zone 2 difference curve and a Zone 2-Zone3 difference curve. Although two curves per plot are preferred, anynumber of curves may be plotted, as desired. The ratio or differencecurves are then analyzed for particular patterns to identify strong orpoor cement bonding throughout the depth of the wellbore (step 145).

FIG. 17 is an illustrative set of amplitude and ratio plots produced byperforming the method of FIG. 16 in a hypothetical wellbore environment,in accordance with embodiments. Specifically, FIG. 17 shows a ratiocurve plot 150 and an amplitude cement bond log 151. The ratio curveplot 150 includes two ratio curves—a Zone 1-to-Zone 2 curve 148, and aZone 2-to-Zone 3 curve 149. These curves are plotted as a function ofdepth along the vertical axis. The log 151 is similar to the logs shownin FIG. 13 in that it shows grayscale-coded average absolute valueamplitude data for each of four zones at a plurality of depths. Inparticular, log 151 comprises sub-logs 151 a-151 d, which correspond tohypothetical Zones 1-4, respectively. Comparing the plot 150 to the log151 in this manner facilitates the initial interpretation of the plot150. After the behavior of plot 150 is understood by interpreting it inlight of log 151, the log 151 is no longer necessary and the plot 150may be interpreted and used independently of log 151.

The behavior of the ratio curves shown in plot 150 indicates the qualityof cement bond present in the annular space behind the second casingstring in a multi-string wellbore environment. The determination as towhich zone ratios should be calculated and plotted rests at least inpart on the bond of Zone 1 (i.e., the inner casing string). If theamplitude curves associated with Zone 1 suggest that Zone 1 is freepipe—i.e., that Zone 1 contains an inner casing string that is notbonded to cement in the annular space adjacent to the string—then ratiosfrom any two zones may typically be used to calculate and plot a ratiocurve (and, alternatively, differences from any two zones may typicallybe used to calculate and plot a difference curve). If, however, theamplitude curves associated with Zone 1 suggest that Zone 1 is bonded tocement, then Zone 1 preferably is not used in ratio or differentialcalculations.

The ratio curves 148, 149 in plot 150 converge and diverge. In someinstances, they converge to the point that they are the same value. Ascan be seen by comparing the behavior of the ratio curves to theamplitude values in log 151, when the ratio curves diverge from eachother at a particular depth, the amplitude values in logs 151 a-c (i.e.,Zones 1-3) decrease at that same depth. As explained in detail above,decreasing amplitude values indicate strengthening cement bond. Thus,for instance, at depth x1, the ratio curves are highly divergent, andrelatively low amplitudes (indicating strong bond) are seen in Zones 1-3at the same depth in log 151. At depth x2, the ratio curves sit atop oneanother and are effectively the same value; at this same depth in log151, relatively high amplitudes (indicating poor bond) are seen in Zones1-3. At depths x3-x5, the divergent ratio curves in plot 150 correspondto relatively low amplitudes (and strong bonds) at the same depths inlog 151. Depths x1-x5 are selected merely for illustrative purposes. Therelationships described between plot 150 and log 151 at depths x1-x5 aregenerally valid for all depths. The behavior of difference curves issimilarly predictive of the cement bond quality at any given depth.

FIG. 18 is another illustrative set of amplitude and ratio plotsproduced by performing the method of FIG. 16 in a hypothetical wellboreenvironment, in accordance with embodiments. The plot 152 contains tworatio curves—Zone 1-to-Zone 3 curve 154 and Zone 2-to-Zone 3 curve 155.Log 153 contains four sub-logs 153 a-d, which correspond to hypotheticalZones 1-4, respectively. The relationships described above with respectto FIG. 17 are also valid in FIG. 18. Specifically, the more the ratiocurves in plot 152 diverge, the lower the amplitudes at the same depthfor Zones 1-3 in log 153, thus indicating a strong cement bond at thatdepth. Similarly, the more the ratio curves in plot 152 converge, thehigher the amplitudes at the same depth for Zones 1-3 in log 153, thusindicating poor cement bond at that depth. For instance, at depth x1,the ratio curves diverge significantly. At the same depth in log 153,the grayscale coding indicates low amplitudes and, therefore, strongcement bond. At depth x2, however, the ratio curves have fullyconverged. Thus, at the same depth in log 153, the grayscale codingindicates high amplitudes and, therefore, a poor cement bond. Thebehavior of difference curves is similarly predictive of the cement bondquality at any given depth.

Analyzing the behavior of ratio or difference curves in this mannerpresents multiple advantages. First, an interpreter inspecting a ratiocurve plot is able to determine, at a glance, the relative cement bondquality at any given depth. For instance, if an interpreter wishes toidentify the depths in a hypothetical wellbore with the poorest cementbonding, he may simply inspect a ratio curve plot—such as plot 150 or152—and identify the areas where the ratio curves have identical values.Similarly, if the interpreter wishes to identify the depths with thestrongest cement bonding, he may identify the depths in the ratio curveplot at which the ratio curves diverge most significantly. Thisease-of-use is enhanced by the fact that a ratio curve plot has fewercurves (and is thus easier to read and interpret) than an equivalentamplitude plot, since a single ratio curve is formed using amplitudedata from multiple amplitude curves.

Another advantage to the use of ratio or difference curves is thatbecause such curves are inherently relative in nature, they suffer to alesser degree from environmental conditions that may distort discreteamplitude measurements. More specifically, in some downhole environmentscontaining multiple concentric casing strings, the presence of an innerstring (and fluid or cement in the annulus outside the inner string) mayinterfere with obtaining accurate amplitude values of waves receivedfrom a cement sheath outside the second casing string. For instance,analyzing amplitude values from a hypothetical Zone 6 (corresponding toan outer cement sheath) necessarily implicates the use of data that hasbeen subject to interference arising from the inner string. However,when amplitude values from Zone 6 are analyzed relative to values fromZone 4 (by calculating ratios or differences between the two zones), theimpact of the interference caused by the inner string (and fluid orcement outside the inner string) is attenuated, because both the datafrom Zones 4 and the data from Zone 6 have been subject to the same orsimilar levels of interference. By taking ratios or differences of datafrom the two zones, the interference effects are dampened.

Numerous other variations and modifications will become apparent tothose skilled in the art once the above disclosure is fully appreciated.It is intended that the following claims be interpreted to embrace allsuch variations, modifications and equivalents. In addition, the term“or” should be interpreted in an inclusive sense.

The present disclosure encompasses numerous embodiments. At least someof these embodiments are directed to a method for evaluating a cementsheath in a wellbore that comprises transmitting sonic or ultrasonicwaves from a logging tool disposed in a wellbore;

receiving reflected waves at the logging tool and recording waveformsbased on the received waves; processing the waveforms to determineaverage absolute value amplitude data for each of a plurality of zones;and determining a number using average absolute value amplitude data fora first of the plurality of zones and average absolute value amplitudedata for a second of the plurality of zones. Such embodiments may besupplemented in a variety of ways, including by adding any of thefollowing concepts or steps, in any sequence and in any combination: thenumber is a ratio or a difference; determining a first set of additionalnumbers using average absolute value amplitude data for said first ofthe plurality of zones at multiple depths and average absolute valueamplitude data for said second of the plurality of zones at saidmultiple depths; determining a second set of additional numbers usingaverage absolute value amplitude data for said first of the plurality ofzones at said multiple depths and average absolute value amplitude datafor a third of the plurality of zones at said multiple depths; plottingsaid first and second sets of additional numbers on a common plot;analyzing said common plot to identify areas where a first curve for thefirst set of additional numbers diverges from a second curve for thesecond set of additional numbers; analyzing said common plot to identifyareas where a first curve for the first set of additional numbersconverges with a second curve for the second set of additional numbers;comparing said common plot to average absolute value amplitude dataplotted on a color-coded, grayscale-coded or intensity-coded log;processing the waveforms to determine average absolute value amplitudedata comprises determining said average absolute value amplitude data atminima and maxima of the waveforms; and identifying said minima andmaxima of the waveforms by calculating derivatives of the waveforms.

At least some of the embodiments disclosed herein are directed to amethod for evaluating a cement sheath in a multi-string wellbore thatcomprises lowering a logging tool into a wellbore; transmitting sonic orultrasonic signals from the tool; receiving reflected signals andrecording one or more of said signals as a waveform; determining a setof absolute value amplitudes of the waveform at minima and maxima ofsaid waveform; dividing said set of absolute value amplitudes into aplurality of zones; determining a mean absolute value amplitude for eachof the plurality of zones; and calculating a number using mean valuescorresponding to different ones of said zones. Such embodiments may besupplemented in a variety of ways, including by adding any of thefollowing concepts or steps, in any sequence and in any combination: thenumber is a ratio or a difference; each of the plurality of zonescorresponds to a different area of the wellbore; calculating a secondnumber using other mean values that only partially overlap with the meanvalues used to calculate said number; and plotting said number and saidsecond number on a plot.

At least some of the embodiments disclosed herein are directed to asystem for evaluating cement bonding in a wellbore that comprises alogging tool that transmits sonic or ultrasonic waves, receivesreflected waves, and records waveforms based on the received waves; andprocessing logic coupled to the tool that determines a set of meanabsolute value amplitudes of a waveform at minima and maxima of saidwaveform, said mean values divided into different zones, wherein theprocessing logic determines a number using mean values from multiple ofsaid different zones. Such embodiments may be supplemented in a varietyof ways, including by adding any of the following concepts, in anysequence and in any combination: the number comprises either a ratio ora difference; the processing logic further determines an additionalnumber using mean values from any two of said different zones, whereinthe processing logic compares the number and the additional number todetermine a trait of a cement sheath disposed in said wellbore; and toidentify said minima and maxima, the processing logic determines aderivative of said waveform.

What is claimed is:
 1. A method for evaluating a cement sheath in awellbore, comprising: transmitting sonic or ultrasonic waves from alogging tool disposed in a wellbore; receiving reflected waves at thelogging tool and recording waveforms based on the received waves;processing the waveforms to determine average absolute value amplitudedata for each of a plurality of zones; calculating a difference betweenan average absolute value amplitude data for a first of the plurality ofzones and an average absolute value amplitude data for a second of theplurality of zones to get a value; and generating a composite image ofthe cement sheath in the wellbore, wherein the composite image is basedat least in part on the difference between the average absolute valueamplitude data for the first of the plurality of zones and the averageabsolute value amplitude data for the second of the plurality of zones.2. The method of claim 1, further comprising: determining a first set ofadditional values using average absolute value amplitude data for saidfirst of the plurality of zones at multiple depths and average absolutevalue amplitude data for said second of the plurality of zones at saidmultiple depths; and determining a second set of additional values usingaverage absolute value amplitude data for said first of the plurality ofzones at said multiple depths and average absolute value amplitude datafor a third of the plurality of zones at said multiple depths.
 3. Themethod of claim 2, further comprising plotting said first and secondsets of additional values on a common plot.
 4. The method of claim 3,further comprising analyzing said common plot to identify areas where afirst curve for the first set of additional values diverges from asecond curve for the second set of additional values.
 5. The method ofclaim 3, further comprising analyzing said common plot to identify areaswhere a first curve for the first set of additional values convergeswith a second curve for the second set of additional values.
 6. Themethod of claim 3, further comprising comparing said common plot toaverage absolute value amplitude data plotted on a color-coded,grayscale-coded or intensity-coded log.
 7. The method of claim 3,further comprising: analyzing said common plot to identify areas where afirst curve for the first set of additional values diverges from asecond curve for the second set of additional values; and analyzing saidcommon plot to identify areas where a first curve for the first set ofadditional values converges with a second curve for the second set ofadditional values.
 8. The methods of claim 1, wherein processing thewaveforms to determine average absolute value amplitude data comprisesdetermining said average absolute value amplitude data at minima andmaxima of the waveforms.
 9. The method of claim 8, further comprisingidentifying said minima and maxima of the waveforms by calculatingderivatives of the waveforms.
 10. A method for evaluating a cementsheath in a multi-string wellbore, comprising: lowering a logging toolinto a wellbore; transmitting sonic or ultrasonic signals from thelogging tool; receiving reflected signals and recording one or more ofsaid signals as a waveform; determining a set of absolute valueamplitudes of the waveform at minima and maxima of said waveform;dividing said set of absolute value amplitudes into a plurality ofzones; determining a mean absolute value amplitude for each of theplurality of zones; calculating a difference between mean valuescorresponding to the plurality of zones to get a value; and generating acomposite image of the cement sheath in the multi-string wellbore,wherein the composite image is based at least in part on the differencebetween mean values corresponding to the plurality of zones.
 11. Themethods of claim 10, wherein each of the plurality of zones correspondsto a different area of the wellbore.
 12. The methods of claim 10,further comprising calculating a second value using other mean valuesthat only partially overlap with the mean values used to calculate saidvalue.
 13. The method of claim 12, further comprising plotting saidvalue and said second value on a plot.
 14. A system for evaluatingcement bonding in a wellbore, comprising: a logging tool that transmitssonic or ultrasonic waves, receives reflected waves, and recordswaveforms based on the received waves; processing logic coupled to thetool that determines a set of mean absolute value amplitudes of awaveform at minima and maxima of said waveform, said mean values dividedinto different zones, wherein the processing logic determines adifference between mean values from said different zones; and a cementsheath log, wherein the cement sheath log displays a composite image,wherein the composite image is based at least in part on the differencebetween mean values from said different zones to get a value.
 15. Thesystems of claim 14, wherein the processing logic further determines anadditional value using mean values from any two of said different zones,and wherein the processing logic compares the value and the additionalvalue to determine a trait of a cement sheath disposed in said wellbore.16. The systems of claim 14, wherein, to identify said minima andmaxima, the processing logic determines a derivative of said waveform.17. The systems of claim 14, wherein the processing logic furtherdetermines an additional value using mean values from any two of saiddifferent zones, wherein the processing logic compares the value and theadditional value to determine a trait of a cement sheath disposed insaid wellbore, and wherein, to identify said minima and maxima, theprocessing logic determines a derivative of said waveform.